1. Field of the Invention
The present invention relates to missiles and rocket motors. More specifically, the present invention relates to rocket motor nozzles.
2. Description of the Related Art
Rocket motors typically create thrust by expelling a high-temperature exhaust produced by the combustion of solid or liquid propellants through a nozzle. The hot gas (or liquid or plasma) exhaust exits from the combustion chamber through a narrow opening (or “throat”) into the nozzle. The nozzle is shaped such that it causes the gas to expand and accelerate, converting the thermal energy into kinetic energy. As the gas expands, it exerts pressure against the walls of the nozzle, forcing the missile in one direction while the gas accelerates in the opposite direction.
Missile propulsion airframe technologies today typically rely on separate metallic reinforced dome and nozzle assemblies fabricated with numerous special use laminates for thermal protection, primary airframe, and subcomponent assembly capabilities. A typical dome/nozzle assembly includes a structural shell for providing structural support usually made from a metal such as aluminum or steel, plus several layers of insulation. Exhaust plume temperatures can reach up to about 5000° F., which is much higher than the melting point of the materials traditionally used to form the structural shell of the dome/nozzle assembly. In order to prevent the dome and nozzle from melting, the metal shell is typically protected by one or more layers of high melting point insulation materials—such as silica, glass, or carbon phenolics—and/or ablative materials designed to erode in a controlled manner.
The nozzle throat area, which is subject to the highest temperature loads, typically includes an insulation layer made from a very high melting point material such as carbon-carbon. Carbon-carbon, however, has a very low thermal coefficient of expansion (TCE) while the metal shell has a very high TCE. The carbon-carbon insulation therefore cannot be bonded directly to the steel shell, since the large TCE differential could cause the bond to break when heated. Several layers of materials having different TCEs are therefore typically placed between the insulation and the steel shell to gradually increase the TCE.
These multiple insulation layers, however, result in multiple bond joints that may come apart when subject to extreme heat loads. This can become a problem particularly for pulsed rocket motors. A pulsed rocket motor includes multiple segments of propellant separated by a barrier. Each segment is ignited separately, with periods between segments in which no propellant is burned (during which the missile coasts). Pulsed rocket motors can offer increased range and efficiency, but typically create increased thermal stresses on the rocket motor nozzle. After a propellant segment is burned through, the heat from the exhaust diffuses through the multiple layers of the dome/nozzle assembly, weakening the bond joints. In a conventional non-pulsed motor, this is usually acceptable because the nozzle is no longer needed; the nozzle only needs to remain intact during the single burn period. In a pulsed motor, however, the dome/nozzle assembly must maintain structural integrity through one or more additional burn periods. This can be very difficult to achieve with conventional dome/nozzle designs.
In addition, the multiple laminate interfaces with differing materials found in traditional rocket motor aft bodies require thick bond joints, o-rings, gaskets, and seals to achieve a thermal shock capability during propulsion ignition and burn, all involving significant manual labor, structural weight, assembly cost and complexity.
Hence, a need exists in the art for an improved rocket motor nozzle that offers improved thermal performance, less structural weight, and lower fabrication cost than prior approaches.